Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink define the printing image areas and the ink-rejecting areas define the background areas.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy inks in the photo-exposed (negative-working) or in the non-exposed areas (positive-working) on a hydrophilic background.
In the production of common lithographic printing plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used.
Upon imagewise exposure of the light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
Alternatively, printing plates are known that include a photosensitive coating that upon image-wise exposure is rendered soluble at the exposed areas. Subsequent development then removes the exposed areas. A typical example of such photosensitive coating is a quinone-diazide based coating. they show a lower dot crispness. The trend towards heat mode printing plate precursors is clearly seen on the market.
EP-A-444 786, JP-63-208036,and JP-63-274592 disclose photopolymer resists that are sensitized to the near IR. So far, none has proved commercially viable and all require wet development to wash off the unexposed regions. EP-A-514 145 describes a laser addressed plate in which heat generated by the laser exposure causes particles in the plate coating to melt and coalescence and hence change their solubility characteristics. Once again, wet development is required.
EP-A-580 393 disclose a lithographic printing plate directly imageable by laser discharge, the plate comprising: (a) a topmost first layer; and (b) a second layer underlying the first layer; wherein the first layer is characterized by efficient absorption of infrared radiation; and (d) the first and second layer exhibit different affinities for at least one printing liquid selected from the group consisting of ink and an abhesive fluid for ink. According to this invention printing plates for wet offset and printing plates for waterless offset can be prepared. However the plates for wet offset have a poor endurance.
WO 94/18005 discloses a heat mode recording material having a high recording speed comprising on a support having an ink receptive surfaceor being coated with an ink receptive layer a substance capable of converting light into heat and a hardened hydrophilic surface layer having a thickness not more than 3 .mu.m. The substance capable of converting light into heat is present in the support or in a separate recording layer. This is detrimental for a high sharpness of the image due to lateral diffusion of the heat and the light diffraction.